The present invention relates to an apparatus for the electrofiltration of dust and other small particulate contaminants from a gaseous carrier material.
A variety of filtration devices are used to remove particulate contaminates, including dust particles, mists, smoke particles and the like from gaseous carrier materials, and particularly from air (hereinafter collectively referred to as xe2x80x9cairxe2x80x9d). Certain of these filter devices rely on particle capture based on charges inherently or actively induced on the particles. With the active charge devices generally there is a charge emitter or ionizer that actively transfers charges to the particles. A collection device is coupled with the charging device to capture the charged particles. These electrostatic air filters have demonstrated improved collection efficiencies for small particulate materials as compared to conventional mechanical filtration devices.
Electrofilters are widely used today for industrial gas cleaning in the removal particles smaller than 20 microns. Electrofilters employ ionization or other charge emitting sources and forces from electric fields to promote the capture of particles in high flow-through, low pressure drop systems. The electrofilters can be either a single-stage device, wherein the ionization source and collection electrode are combined in a single element, or more commonly a two-stage device that employs an upstream ionization source that is independent of a down stream particle collection stage. Functional attributes such as relatively high efficiency and low pressure drop make two-stage electrofilters particularly well suited for in-door air quality enhancement applications. However these devices are relatively expensive, require periodic cleaning and can become odorous over time. The collector performance is also negatively impacted by the deposited particles and can deteriorate over time.
In two stage electrofilter devices, particulates are generally charged as the particulate-laden gas stream is passed between a high-voltage electrode and a ground that are maintained at a field strength sufficient to establish a glow discharge or corona between the electrodes. Discharged gas ions and electrons generated in the corona move across the flow stream, colliding with and charging particulate contaminants in the gas stream. This mechanism, which is known as bombardment or field charging, is principally responsible for charging particles greater than 1 micron in size. Particulates smaller than about 0.2 microns are charged by a second mechanism known as diffusion charging, that results from the collection of gas ions on particles through thermal motion of the ions and the Brownian motion of the particles.
If a dielectric or conductive particle is placed in the path of mobile ions a proportion of the surface of each particle will be given a strong electrical charge. That charge is redistributed over the surface of a conductive particle almost instantaneously whereas it is only very slowly redistributed over the surface of a non-conductor particle. Once charged, particulate contaminants are moved toward the collector surface as they enter the particle collection stage. In the absence of mobile ions, conductive particles captured on the collector surface are free to leave the surface because they have shared their charge with the surface. On the other hand, dielectric and/or non-conducting particles that do not readily lose their charge are retained on the collector surface. This attraction force weakens, however, as layers of particles build up and, in effect, create an electrical insulation boundary between particles and the collector surface. These charge decoupling mechanisms, in combination with flow-stream induced dynamic motion at the collector surface, can lead to disassociation of particulate materials from the collector. Once disassociation from the collector surface occurs, the particle is free to reentrain itself in the air stream.
Electrofiltration devices that rely on electrostatic attraction between contaminant particles and charged collector surfaces are generally exemplified by collectors formed from actively charged conductive (metallic or metalized) flat electrode plates separated by dielectric insulators such as described in U.S. Pat. Nos. 4,234,324 (Dodge, Jr.) or 4,313,741 (Masuda et.al.). With these devices, inherently charged particles, or particles induced with a charge, such as by an ionizer or charge emitter as described above, are passed between flat charged electrode collector plates. Dodge proposes use of thin metalized Mylar sheets separated by insulating spacers on the ends of the sheets and wound into a roll. These constructions are described as lower cost than conventional metal plates and can be powered by low voltage sources, which, however, require closer spacing of the metalized sheets. This construction allegedly is of a cost that would permit the collector to be discarded rather than requiring periodic cleaning. Additionally, this construction would also eliminate the odor problem. Masuda et.al. also describes the above problems with conventional metal plates and proposes a specific plate design to address the problems of sparking and some of the loss in efficiency problems, but periodic cleaning is still required and odors are still a problem.
In an effort to provide serviceable electrofiltration devices that do not require periodic cleaning, U.S. Pat. No. 3,783,588 (Hudis) describes the use of films of permanently electrically charged polymers that move on rolls into and out of the collector. In this construction, new, uncontaminated, charged film is constantly moved from one roll into the collector space and dirty film is moved out of the collector space onto a collector roll. Periodically the film rolls must be replaced, which would be time consuming, particularly where large numbers of film rolls are employed. There still remains a need for low cost, modular, disposable collector devices that exhibit high collection efficiencies.
The electrofiltration apparatus of the invention comprises an ionization stage and a particle collection stage. The particle collection stage comprising a collector cell formed of at least one flow channel layer formed by at least one structured film layer and a second layer. The structured film layer has a first face and a second face, at least one face of the structured film forms, at least in part, flow channels and has high aspect ratio structures over at least a portion of the face forming the flow channels. A second film layer comprising the flow channel layer second layer, or a further layer, at least in part, defines fluid pathways through the flow channels of the collector cell. The film layers are electret charged. At least one film layer forming the flow channels in the collector cell is a contoured film in a preferred embodiment.